Some predefined functions do not obey this
rule, in particular the structural equality function (=) which is
polymorphic (its type is 'a -> 'a -> bool) but which explores the
structure of its arguments to test their equality.

Translator's note: In Molière's play Le
Bourgeois Gentilhomme (The Bourgeois Gentleman), the character Mr.
Jourdain is amazed to discover that he has been speaking prose all his
life. The play can be found at